Well, I printed out a replacement for the idler bracket and installed the tensioner. It works great but I'm somewhat disappointed because it's tipped because of the tension, even though the idler bracket is a reasonably tight fit. It would be really easy to over-tighten and break the Perspex so a measure of caution is due. I've ran a test print but it looks like I've damaged my controller board. Maybe too much radial stress on the Y axis fried a MOSFET? I'll troubleshoot later today.